The invention relates to a magnetic field sensor comprising a transducer element. Such sensors may be employed inter alia:
as magnetic heads, which can be used to decrypt the magnetic flux emanating from a recording medium in the form of a magnetic tape, disc or card;
in compasses, for detecting the terrestrial magnetic field, e.g. in automotive, aviation, maritime or personal navigation systems;
in apparatus for detecting position, angle, velocity or acceleration, e.g. in automotive applications;
as field sensors in medical scanners, and as replacements for Hall probes in various other applications;
as current detectors, whereby the magnetic field produced by such a current is detected.
Sensors as specified in the opening paragraph are well known in the prior art. The transducer element in such sensors typically comprises a magneto-resistance element, which translates magnetic flux variations into a correspondingly fluctuating electrical resistance R; a measure of the performance of the element is then expressed in the so-called magneto-resistance (MR) ratio, which quantifies the maximum change in R as a function of applied magnetic field. Sensors of this type may be based on one of the following effects:
The Anisotropic Magneto-Resistance effect (AMR), whereby R in a magnetic body is dependent on the orientation of the body""s magnetization with respect to the direction of electrical current flow through the body; or
The Giant Magneto-Resistance effect (GMR), whereby R is determined by the relative orientation of the magnetization vectors in two distinct magnetic bodies, for example:
two layers which are sandwiched about an interposed metallic layer (interlayer), thus forming a so-called spin-valve trilayer (see, for example, the elucidation given by B. Dieny et al in U.S. Pat. No. 5,206,590 and J. Magn. Magn. Mater. 136 (1994), pp 335-359);
a multilayer comprising a plurality of stacked F/M bilayers, in which F is a ferromagnetic layer and M is a metallic layer, neighboring F-layers being antiferromagnetically coupled across intervening M-layers.
A disadvantage of known sensors based on AMR and GMR is that they demonstrate a relatively small MR ratio. Typically, the room-temperature MR values for AMR sensors are of the order of about 2%, whereas those for practical GMR sensors are generally of the order of about 5-10% at best. Consequently, such conventional sensors are relatively insensitive.
It is an object of the invention to provide a more sensitive magnetic field sensor. In particular, it is an object of the invention to provide a sensor which exploits a magneto-resistance effect with a room-temperature MR ratio of the order of 15% or more. Moreover, it is an object of the invention that such a sensor should be relatively compact, entailing an efficient use of materials and space.
These and other objects are achieved according to the invention in a magnetic field sensor comprising a transducer element, characterized in that:
I. the transducer element is a Spin Tunnel Junction, comprising a first and second magnetic layer which are sandwiched about an interposed electrical insulator layer (interlayer);
II. the sensor comprises a yoke having two arms;
III. the first magnetic layer is in direct contact with a first portion of a first arm of the yoke.
The principles of Spin Tunnel Junctions (STJs) are discussed in detail in an article by J. C. Slonczewski in Phys. Rev. B 39 (1989), pp 6995-7002, and a study of the properties of a particular STJ is presented in an article by S. S. P. Parkin et al. in J. Appl. Phys. 81 (1997), 5521. Because the STJ contains electrically insulating material (its interlayer) instead of purely metallic material, the principle of operation of an STJ is radically different to that of conventional AMR or GMR elements. For example, in a GMR element, the electrical resistance is metallic, and is mediated by spin-dependent scattering effects; on the other hand, in an STJ, the electrical resistance is mediated by spin-dependent tunneling effects. Another difference is that, in a (practical) AMR or GMR element, the measurement current is directed parallel to the plane of the element; on the other hand, in an STJ, the measurement current must be directed (tunneled) across the interlayer, and so is directed perpendicular to the plane of the element. These differences help account for the most dramatic advantages of an STJ: because of the STJ""s high tunnel resistance, the measurement current can afford to be very small (of the order of 1 about xcexcA, or less), and the room-temperature, low-field MR-ratio of an STJ is routinely of the order of at least 15%.
The term xe2x80x9cmagnetic layerxe2x80x9d as used with reference to an STJ should be broadly interpreted. Such a magnetic layer may, for example, be comprised of one of the following:
a single layer of ferromagnetic material;
a ferromagnetic film which is accompanied by a thin, metallic, non-magnetic film on the side adjacent to the nearest yoke-arm;
two ferromagnetic films which are exchange-coupled across an interposed electrically conducting film;
a ferromagnetic film which is arranged in a stack with a pinning structure (examples of which are given herebelow in Embodiment 1), the pinning structure serving to directionally fix the magnetization in the adjacent ferromagnetic film.
In all cases, it is important to realize that the magnetic layer does not contain any electrically insulating films; the only electrically insulating structure in the STJ is the tunnel barrier (interlayer) between the first and second magnetic layers.
When a AMR or GMR transducer element is employed in a yoke-type magnetic field sensor, the element is electrically insulated from the yoke, e.g. by the use of a so-called separation-oxide layer between the element and the yoke; this is to prevent the yoke-arm from acting as an electrical shunt around the transducer element (in which, as has already been explained, the measurement current is parallel to the plane of the element and also to the top surface of the yoke-arm). However, the presence of an insulating layer between the yoke and the transducer element reduces the magnetic contact between the two, which accordingly reduces the efficiency of the sensor. This acts as a deterrent to the use of a yoke in conjunction with conventional sensors. In contrast, the inventors have realized that, when an STJ is employed instead of a conventional magneto-resistance transducer element, the use of a yoke becomes a more viable possibility. This is because the measurement current through the STJ is directed perpendicular to its plane, so that a yoke-arm in electrical contact with one of the magnetic layers of the STJ does not act as an electrical shunt around the transducer; the presence of a special separation-oxide layer between the STJ and the yoke is thus unnecessary. For this reason, the invention stipulates that the STJ be in direct contact with the yoke, thereby guaranteeing good magnetic contact and optimal efficiency. Moreover, the yoke-arm which is in contact with the magnetic layer of the STJ also serves as an electrical contact to that magnetic layer, which alleviates the need to provide electrical contact via a separate lead. In addition, the absence of a separation-oxide layer reduces the quantity of materials required in the sensor, simplifies its manufacturing procedure, and allows it to be more compact.
The yoke-type magnetic field sensor according to the invention is particularly advantageous when employed as a contact magnetic head, e.g. when reading magnetic tape or a hard disc. This is because it is then the relatively durable yoke which makes contact with the recording medium, instead of the relatively fragile transducer element. Apart from an advantage in terms of mechanical wear, this configuration additionally leads to reduced thermal noise.
In an advantageous embodiment of the sensor according to the invention, the said first portion of the first arm of the yoke constitutes the first magnetic layer of the STJ, i.e. the first yoke-arm plays the role of first magnetic layer in the STJ. In such an embodiment, the first yoke-arm does not contain a magnetic gap underneath the STJ, but is instead continuous. This embodiment therefore has the advantage that:
it is even more compact and economic, since a distinct first magnetic layer is not required in addition to the yoke;.
it is easier to manufacture, since a magnetic gap does not have to be created in the employed yoke.
In an embodiment suitable for use in extremely small sensors (i.e. sensors for which the so-called characteristic length is very small), a second portion of the second arm of the yoke constitutes the second magnetic layer. Such an embodiment is even more compact, since the different arms of the yoke now play the role of both the first and second magnetic layers. In this latter embodiment, it is important that the two yoke-arms be electrically insulated from one another, so as to prevent the formation of a short circuit across the STJ.
A further refinement of the first embodiment in the previous paragraph is characterized in that the thickness t1 of the first portion of the first arm of the yoke is less than the thickness of the rest of the first arm immediately adjacent thereto. By locally thinning the first arm in this manner, magnetic flux in the first portion becomes more concentrated, thus serving to increase the sensitivity of the sensor. This effect is increased even further if the thickness t2 of the second portion of the second arm of the yoke is also less than the thickness of the rest of the second arm immediately adjacent thereto; in that case, magnetic flux also becomes more concentrated in the second portion, causing a further increase in sensitivity of the sensor.
The skilled artisan will immediately appreciate that, if the STJ is to be useful as a sensor, the respective magnetizations M1 and M2 in the first and second magnetic layers must change their relative orientation as a function of applied magnetic field. This can, for example, be achieved by employing different magnetic materials in the two layers, or by ensuring that M1 and M2 are mutually perpendicular in the quiescent state (e.g. using exchange biasing). As an alternative, a particular rendition of the embodiments described in the previous paragraph is characterized in that t2 greater than t1. In such an embodiment, the discrepant values of t1 and t2 result in different flux concentrations in the first and second yoke-arms, respectively, so that, when a given external magnetic field is offered to the yoke, M1 and M2 will rotate to different extents. Good results are achieved for sensors in which the value of t2/t1 lies in the range 2-30, with particularly good results at t2/t1≈10.
In addition to the transducer and the yoke, the sensor according to the invention may comprise various other structures. For example:
in the case whereby only one of the magnetic layers of the STJ is in contact with the yoke, the other magnetic layer of the STJ will have to be provided with an electrical contact lead;
a test/biasing conductor may be provided (e.g. as illustrated in FIG. 4).